The state says there has been significant progress made to expand cellular phone coverage in Vermont over the past few years. Yet it’s hard to tell how much progress has been made and what remains to be done.

That’s why Mike Ray and Jo Wilson are spending their days driving the roads of Vermont this month in a vehicle with two Coleman coolers mounted on the roof.

Often in their work they use fairly high-tech gear when measuring radio frequencies for broadcast stations, emergency responders and wireless providers.

But for measuring cellphone signals, it turns out the best piece of equipment is a regular cellphone. Ray and Wilson have five of them mounted in makeshift racks inside the two green coolers atop his rental car.

The two work for a Colorado-based company called Pericle Communications which has been contracted by the state, using a federal grant to map cellphone coverage in Vermont.

They’re covering more than 5,500 miles of Vermont roads, measuring cellular signal strength and connection speeds, and determining where cellphone providers have service.

Like a group of busy teenagers, the phones on the roof are constantly receiving and sending data and making calls. The coolers keep them from overheating.

Ray said he could do this with computer modeling, but a map created that way would only show theoretical coverage.

“Really the only way to know is to go drive it,” he said. “There’s a reason why you have the Verizon guy wandering out with a phone asking, ‘Can you hear me now?’ It is the only way to tell.”

Ray’s company mapped cell coverage in Vermont once before, in 2010.

Since then there’s been significant progress in expanding cell coverage, according to the Vermont Telecommunications Authority. But without the maps, it’s difficult to quantify the gains and chart a course for improving coverage.

Chris Campbell, VTA’s executive director, said as broadband service expands Vermonters are increasingly able to use available technology to access a cellphone network through their Internet connection.

“What that doesn’t necessarily help is for people who are either traveling from place to place or in places where people congregate outside of where they live and work,” Campbell said. “When we focus on the cellular specific mission that’s really what we focus on.”

The state’s goal for cellphone coverage is to ensure the major carriers are providing service to all of Vermont’s interstates and well-traveled state highways.

No one is hazarding a guess as to when that will be, but Vermont is using grants and other incentives to convince cellular companies to fill in the gaps in service. The new map will show them just where those gaps are.